1. Field of the Invention
The present invention relates to improvement of the slipperiness (free of interfacial sticking during winding operation) of polyester films transported (winding) at low speed under low tension, and the polyester film of the present invention is useful as the base-film of a magnetic recording tape for a video tape recorder (VTR) or a computer data storage system.
2. Summary of Prior Art
The magnetic recording tape for VTR use is transported at low speed (0.5-40 cm/sec) under low tension (5-50 grams). Therefore, the tape is required to have an exceedingly high slipperiness and an excellent abrasion resistance.
To improve the slipperiness of a film, it is effective to make the surface of the film microscopically uneven; and the known techniques for that purpose are, for example, incorporation of inorganic fine particles in the polymer of a raw material of the film, and eduction of insoluble residues of catalyst in the polymer.
Abrasion resistance is necessary for magnetic recording tapes to endure repeated uses; and the abrasion may be classified into two categories, fatigue abrasion and frictional one. The former results from the continued movement of polyester film-based magnetic tape during use. Movement of a magnetic tape in contact with a tape transport abrades the uncoated side of the magnetic tape with the resultant formation of fragments thereof (debris), which results in temporary and permanent errors in the reading of the information stored on the magnetic oxide coating. This occurs as a result of wind-up of the tape. The debris on the surface of the uncoated side is transferred and deposited on the magnetic layer (coated side) of the tape, and causes separation of the tape from the head for reading the stored information. This error may be considered as temporary because the debris can be partly removed from the surface of the tape by tape-cleaning, etc. In many instances, however, debris particles are pushed into the magnetic layer during wind-up, and cause indentations in the surface of the tape. Such indentation causes head-to-tape separation resulting in a loss of information.
On the other hand, the frictional abrasion results from the considerable frictional resistance applied to the film during manufacturing or, later, processing of the film prior to coating with magnetic coating, or in the course of the reproduction of information. Polyester debris particles are formed on the uncoated surface of the film in the form of white powders, when a polyester film is being passed over rolls rotating at different speeds, or passed over stationary rods or stationary guides. For example, if the surface to be coated with magnetic coating contains debris, the debris is coated, and the portion of the film surface covered with debris remains uncoated. This coated debris results in a nonuniform tape surface. Nodules, formed by the debris overcoated with the magnetic coating on the otherwise smooth tape surface, may result in a loss of recorded information. For this reason, both fatigue and frictional abrasion of the film should be controlled as low as possible. To reduce the fatigue and the abrasion resistance between film and film, or film and metal, it is necessary to lower the friction coefficient therebetween.
The improvement of the slipperiness of polyester film can be attained by incorporating an insoluble, chemically inert additive having wide particle size distribution in the polymer. For example, U.S. Pat. No. 3,980,611 discloses a method to improve the winding characteristics of a polymer film while keeping its high transparency by incorporating not more than about 0.25% by weight of an inert additive to the polymer, wherein not less than 70% of the total amount of the additive is composed of coarse particles having particle diameters of greater than 2.5 microns and fine particles having particle diameters of less than 1 micron. It has been made clear, however, that although the surface protrusions of the film originated in the additives having particle sizes of greater than 2.5 microns are effective in improving the slipperiness of the film, they give only an unsatisfactory result in the improvement of the abrasion resistance thereof, in case of the low speed transport of the film under low tension. Even by the method for the improvement of the resistance of a film-based magnetic tape to the frictional abrasion and the fatigue resistance, disclosed in U.S. Pat. No. 3,821,156, comprising incorporating fine particles of an inert additive selected from the group consisting of silica and hydrated aluminum silicate together with coarse particles of calcium carbonate [CaCO.sub.3 ] in the polyester, the low speed transport characteristics are found to be poor when the additives contain the particles having particle diameters of greater than 2.5 microns.